Look at this test flight report on UH1:
Investigation...steady state autorotation
What we talk about can be seen in a number of charts. For example these two:
-Figure 2 /page16:
ROD 6700lbs /DA 6000 / 300RPM
-Figure 3/page 17:
ROD 8800lbs /DA6000 / 300RPM
At 6700lbs the minimum rate of descent is 1874fpm, and at 90kts 2200fpm.
At 8800lbs the minimum rate of descent is 1866fpm, and at 90 kts 2100fpm.
At Vy, not a big difference but it is a difference. At 90knots, clearly visible.
Red lines in charts compensated for chart not level by creation on the X- and Y-axes.